The required precision for dimensional measurement is getting higher and higher in semiconductor industries as device sizes are becoming ever smaller. Confocal technology provides a good solution to meet the needs of high-resolution 2D and 3D measurement, among which chromatic confocal technology in particular improves the efficiency of a scanning mechanism for confocal measurement and makes it practicable for real time inspection applications.
In a chromatic confocal system, light rays with different colors are dispersed along an optical axis, so that only a small range of wavelengths can be reflected from an object of interest and then received by an imaging system. The height information can then be determined by analyzing the light spectrum. A spectrum analyzer typically consists of a prism, grating or filters, and intensity sensing devices.
FIG. 1 is a graph showing an exemplary chromatic confocal signal spectrum obtainable from a conventional confocal system. It illustrates a typical chromatic confocal signal which is represented as a spectrum. There is only one peak on the graph indicating that only light that is just in focus can be received by the system, while out-of-focus light from the object surface is eliminated by the pinhole. In the other words, the height of the inspected object surface can be estimated by searching the peak position of the impulse signal or color, wherein the relationship between height and color can be calibrated in advance. In order to determine the entire topography of the object, two-dimensional scanning along the X-Y axes on its surface is required.
To further speed up the chromatic confocal system, a slit may be used instead of a single pinhole point. US Patent Publication number 2010/0188742 entitled “Slit-Scan Multi-Wavelength Confocal Lens Module and Slit-Scan Microscopic System and Method Using the Same” discloses a chromatic or multi-wavelength slit-based scanning system. However, in such a system, the cross-talk effect can be severe as unfocused light may diffuse and spread to neighboring pixels along a direction of the slit, and lower the system's accuracy for depth detection.
U.S. Pat. No. 9,025,245 entitled “Chromatic Confocal Microscope System and Signal Process Method of the Same” discloses a confocal system using a pair of optical fiber modules to modulate a linear or a planar detecting light. It describes splitting the detecting light by passing the light through respective color filters into two different image sensing units. This generates two RGB images, with which the depth or surface profile of the object of interest can be reconstructed according to the ratio of intensity of these two images. One critical drawback of such a system is that merely computing height information from the maximum intensity ratio lacks accuracy since some stray or unfocused light rays can also enter the system causing some intensity offsets and hence altering the ratio of color intensity.
Therefore, it is desirable to improve the accuracy of the light color ratio computation by modulating the spectrum of a light source.